Rectifier 6 of UHF transponder 2 of FIG. 1 is formed by two diodes D1 and D2 and by capacitor C1. Diode D1 is arranged in parallel with diode D2 and capacitor C1 between the input terminals A and B of the integrated circuit. This type of rectifier, in the case of a UHF transponder, allows capacitor C1 to be charged at a voltage close to that defined by the electromagnetic signal received between its two peak values.
If terminal A is defined as the positive terminal and terminal B as the negative terminals, a positive current thus flows in the integrated circuit between terminal A and terminal B. This definition will be maintained hereinafter.
Transponder 2, shown in FIG. 1, has a problem of protection against electrostatic discharge (ESD), more particularly in the case where the discharge current is positive (VA>VB). In fact, in the case of a negative current, diode D1 is conductive such that the current can flow through the diode up to a relatively high value without any damage due to the dissipation of energy, even if diode D1 is of small dimensions. However, in the case of a positive current, when there is electrostatic discharge, diode D1 blocks the passage of the current to its breakdown voltage, for example between 5 V and 10 V in absolute value. Two problems occur in this latter case. First of all, the strong discharge current often destroys circuit 8 before diode D1 allows the current to pass. Then, if the diode is of small dimensions, it cannot absorb power corresponding to the discharge current multiplied by the breakdown current of the diode. Consequently, the transponder is generally destroyed by a positive electrostatic discharge.
In order to obtain an efficient rectifier, diodes D1 and D2 are preferably small Shottky diodes. In order to resolve the aforementioned problem, the transponder 10 shown in FIG. 2 has been proposed, wherein a diode D3 is arranged in parallel with diode D1. This diode D3 has relatively small dimensions with low impedance. Diode D3 has little influence on the rectifier circuit and it is provided with a lower breakdown voltage than that of diode D1, preferably close to the operating voltage of the integrated circuit but higher than the latter.
Transponder 10 has, however, drawbacks. First of all the ESD protection is relatively low since it is difficult to define the breakdown voltage of diode D3 precisely, this voltage being dependent on the integrated circuit manufacturing technology. Another major drawback arises from the fact that diode D3 significantly increases the input capacitance of the integrated circuit, which increases the energy dissipated by the circuit and decreases the voltage at the terminals of supply capacitor C1. The negative consequence of this is a decrease in the communication distance to a given reader.
It is an object of the present invention to propose a transponder of the type described hereinbefore provided with protection against electrostatic discharge (ESD) which does not have the aforementioned drawbacks.